1. Field of the Invention
The present invention relates to a control apparatus for a data storage apparatus, and more particularly to a control apparatus suitable for use in, for example, a main storage apparatus provided with a data hold mode which is capable of utilizing the data hold mode in which a power consumption is low even when the main storage apparatus is being accessed.
2. Related Art
In memory elements constituting a main storage apparatus in a computer or the like, an interruption of voltage supply causes data in the elements to be destroyed. However, known memory elements include one which is provided with a data hold mode such as a static RAM (hereinafter simply referred to as "SRAM"). The data hold mode is defined as a mode in which a supplied voltage is decreased below an allowable operation voltage in a non-select state of the memory elements, wherein data can be held if the supplied voltage presents a voltage value larger than a data hold voltage. In the data hold mode, merely several .mu.A of a power supply current flows, and accordingly the power consumption is extremely low.
The related art has utilized this mode to perform a battery backup in case of system power supply interruption.
The following is a procedure of switching from the system power supply to a battery power supply:
(1) A voltage drop is detected in the system power supply;
(2) The memory elements are set to a non-select state; and
(3) The power supply is switched from the system power supply to the battery power supply (the supply voltage of which is lower than that of the system power supply) to set the storage elements to a data hold mode.
For recovery, after confirming that the system power supply has been turned on, the storage elements are released from the non-select state after a time period required to recover the operation of the storage elements.
As explained above, the data hold mode in which the power consumption is low has been conventionally utilized only when the memory elements are backed up by a battery.
The above-mentioned related art has not considered reduction in the electric power consumed by the main storage apparatus except for the case where it is backed up by a battery. Specifically, when the system power supply is turned on, all memory elements are in either the operating state or the non-select state (hereinafter, the two states are collectively referred to as "the normal mode"), whereby the storage elements are never set in the data hold mode in which the power consumption is low.